In general this invention relates to prosthesis material made of synthetic polymer. More specifically, it relates to an improved method for fabricating prosthesis material, particularly vascular, duct and other conduit material for use with living tissue.
Synthetic prosthesis material has long been the subject of intense investigative effort. The book "Biologic and Synthetic Vascular Prosthesis" edited by James C. Stanley, M.D., published by Grund & Stratton, 1982 provides a complete historic survey of this investigation. Dr. Stanley points out that various materials such as PTFE and polyurethane have been studied and tested. Most of the materials have been found unsatisfactory, particularly for vascular usage, due to the fact that the conduit structures formed of the materials have been, in the case of many materials thrombogenic and, in the cases of other materials such as PTFE, mechanically noncompliant particularly when used in small diameters. Moreover, porosity of the material has been found to be an important requirement which was not satisfactorily met by early materials.
A promising copolyurethane material developed by Lyman et al has been pointed out by Stanley. The Lyman et al material compares in porosity with knitted Dacron material. It is manfactured by liquid polymer precipitation on a mandrel to which an external mesh may be added for tensil strength. The material can be manufactured with varying compliance values by altering the wall density. More detailed information concerning the Lyman et al work is available in a paper by Lyman entitled "Structural Order and Blood Compatibility of Polymeric Prosthesis" which was printed in the IUPAC MACROMOLECULAR SYMPOSIUM, Ciardelli, C. F. & Giusti, P., Eds., Pergammon Press, Ltd., Oxford, 1980, at p. 205 and in the Lyman et al U.S. Pat. No. 4,173,689. The content of all references cited herein are incorporated by reference.
Polyurethane and copolyurethanes have been considered for prosthetic purposes as evidenced by an Annis et al article "An Elastomeric Vascular Prosthesis", Vol. XXIV Trans. Am. Soc. Artif. Intern. Organs, 1978, page 209; an article by Andrade et al entitled "Blood-Materials Interactions - Twenty Years of Frustration"; Volume XXVII, Trans. Am. Soc. Artif. Intern Organs, 1981 page 659 and a comment entitled "Experimental Study of a New Synthetic Vascular Graft" by Gruss et al which appeared at page 518 J. Cardiovas. Surg., 22, 1981 of the XV World Congress of the International Cardiovascular Society.
The synthetic polymer precipitation of these materials as first described by Lyman et al appears to offer the most satisfactory synthetic polymer prosthetic material to date. In that technique, sometimes referred to as a "Mandrel Coating Process" and as indicated above, the lumen of a tubular conduit is formed on a dipped mandrel. The characteristics of the material thus formed are subject to variation depending on the physical and chemical properties of the mandrel. Specifically, the surface characteristics of the mandrel determine in a large part the surface characteristics of the conduit lumen. Also, the wall thickness uniformity of the tubular conduit formed on a mandrel is dependent on selecting a polymer solution viscosity and a dip coating rate that allows film deposition over a significant length of mandrel without excessive running of the solution which results in uneven coating thickness. Furthermore, the polymer concentration is limited in that it is controlled by the type of pore structure desired in the coating.
In this particular technique, for a specific solution viscosity, the dip coating rate must be very exact to achieve a uniform coating on the mandrel. There is a narrow range of polymer solution viscosities which will coat a mandrel uniformly at any dip coating rate.
Although the Lyman et al technique as described in the aforementioned patent is related primarily to block copolymers and more specifically to copolyurethanes, it is generally applicable as is the present invention to any synthetic polymer which is amenable to the precipitation technique utilized. In the technique, a solvent appropriate for the particular synthetic polymer such as N, dimethylformamide for block copolyurethanes is utilized to form a solution of moderate viscosity. A forming device referred to herein as a mandrel or other tool of suitable surface configuration is slowly immersed in the viscous solution and then slowly withdrawn, leaving a solution coating on the mandrel comprising a uniform dispersion of polymer. Voids are introduced, i.e. porosity by extracting the polymer solvent into a miscible solvent in which the polymer is insoluble, resulting in precipitation of the polymer onto the mandrel.
The coated mandrel is exposed to the precipitating solution, which is usually water, by simply dipping the mandrel into it. The process of solvent displacement by the precipitating solution involves a concurrent transfer of precipitating solution into the spaces formerly occupied by the solvent molecules. When the precipitating solution is ultimately removed as by evaporation, the resulting cavities or voids within the polymer structure create a spongy texture, the desired elasticity of which may be matched to living tissue compliance requirements by control over the porosity. By repeating the dipping procedure, the thickness of the polymer coating can be increased and material compliance can be further adjusted to an altered elastic response, if desired.
Of the general class of block copolymers, copolyurethanes, particularly copolyurethane-ureas and copolyether-urethanes are representative of preferred synthetic polymer material for use with the invention and its objective which is the provision of improved synthetic prosthesis material particularly vascular prostheses having a lumen surface of improved smoothness for blood compatibility.
As indicated above, a problem is attendant with the Lyman et al technique wherein the polymer is precipitated on a mandrel in that the surface characteristics of the lumen, i.e. the interior surface of the synthetic vascular conduit material thus formed is determined as to its nature by the solid surface of the mandrel. This has been found to have a pronounced effect not only on its smoothness but on the porosity of the prosthesis material as well.
A pore gradient is also desirable for consistency and strength of the material. That is, it is desirable that the size of the pores should gradually increase through the wall thickness from about 1-2 microns to about 30 microns, for example, on the exterior of the wall to encourage tissue ingrowth. This is not readily obtained by the Lyman et al technique and resort has been taken to multi-coated layers to obtain desired pore size and pore distribution throughout the wall thickness.
On the other hand, this invention provides an improved precipitation technique making possible the formation of improved lumen surface smoothness and desired porosity and in which multi-coated layer formation is not necessary to obtain various wall thicknesses with desired pore size and pore distribution and mechanical compliance.